iMac keeps waking up from Sleep

I have Apple TV box but no Apple TV account. I watch YouTube videos at night instead of TV. When it's time for bed, I like to run my play list and set TV to sleep in ** minutes. I've been doing this for years. Recently, I had the motherboard replaced on iMac. I had to reload everything. Now, the iMac won't go to sleep by itself and if I force it to sleep, it wakes up within 5 minutes. It doesn't matter if I use the shortcut keys, the links or the link from the logged out screen.

It never used to do that. There is nothing new on my computer. No new devices, no new bluetooth devices and no reason for it to wake up until I ask for it.

All fine and dandy but I run YouTube videos in sequence or a movie from external drive through HDMI to my TV which is set to shut off in ** minutes. The videos or movie would end and the computer would go to sleep. Now the computer fails to sleep and the monitor stays on all night. It’s waking up my partner and may cause screen burn.

Shutting down the computer only resolves shutting down the computer but it doesn’t address the “going to sleep mode” and “waking from sleep mode after several minutes”.

I do/did have “Turn display off after 10 minutes” which did work before but doesn’t work anymore.